Breast cancer stem cells and their role in resistance to endocrine therapy

Developmentally, tumours can be viewed as aberrant versions of normal tissues. For example, tumours often retain differentiation markers of their tissue of origin. In addition, there is evidence that they contain cancer stem-like cells (CSCs) that drive tumourigenesis. In this review, we summarise current evidence that breast CSCs may partially explain endocrine resistance in breast cancer. In normal breast, the stem cells are known to possess a basal phenotype and to be mainly oestrogen receptor-α-negative (ER-). If the hierarchy in breast cancer reflects this, the breast CSC may be endocrine resistant because it expresses very little ER and can only respond to treatment by virtue of paracrine signalling from neighbouring, differentiated ER+ tumour cells. Normal breast epithelial stem cells are regulated by the epidermal growth factor receptor and other growth factor receptor signals. The observed increase in growth factor receptor expression in endocrine-resistant breast cancers may reflect a bigger proportion of CSCs selected by endocrine therapies. There is evidence from a number of studies that breast CSCs are ER- and EGR+/HER2+, which would support this view. It is reported that CSCs express mesenchymal genes, which are suppressed by ER expression, further indicating the mutual exclusion between ER+ cells and the CSCs. As we learn more about CSCs, differentiation and the expression and functional activity of the ER in these cells in diverse breast tumour sub-types, it is hoped that our understanding will lead to new modalities to overcome the problem of endocrine resistance in the clinic.

The devil is in the methods: lineage tracing, functional screens and sequencing, hormones, tumour-stroma interactions, and expansion of human breast tumours as xenografts

The meeting of the European Network for Breast Development and Cancer (ENBDC) on 'Methods in Mammary Gland Development and Cancer' has become an annual international rendezvous for scientists with interests in the normal and neoplastic breast. The third meeting in this series, held in April-May 2011 in Weggis, Switzerland, focussed on functional screens and sequencing, hormones, lineage tracing, tumor-stroma interactions and the expansion of human breast tumours as xenografts.

Dickkopf1 regulates fate decision and drives breast cancer stem cells to differentiation: an experimentally supported mathematical model

BACKGROUND

Modulation of cellular signaling pathways can change the replication/differentiation balance in cancer stem cells (CSCs), thus affecting tumor growth and recurrence. Analysis of a simple, experimentally verified, mathematical model suggests that this balance is maintained by quorum sensing (QS).

METHODOLOGY/PRINCIPAL FINDINGS

To explore the mechanism by which putative QS cellular signals in mammary stem cells (SCs) may regulate SC fate decisions, we developed a multi-scale mathematical model, integrating extra-cellular and intra-cellular signal transduction within the mammary tissue dynamics. Preliminary model analysis of the single cell dynamics indicated that Dickkopf1 (Dkk1), a protein known to negatively regulate the Wnt pathway, can serve as anti-proliferation and pro-maturation signal to the cell. Simulations of the multi-scale tissue model suggested that Dkk1 may be a QS factor, regulating SC density on the level of the whole tissue: relatively low levels of exogenously applied Dkk1 have little effect on SC numbers, whereas high levels drive SCs into differentiation. To verify these model predictions, we treated the MCF-7 cell line and primary breast cancer (BC) cells from 3 patient samples with different concentrations and dosing regimens of Dkk1, and evaluated subsequent formation of mammospheres (MS) and the mammary SC marker CD44(+)CD24(lo). As predicted by the model, low concentrations of Dkk1 had no effect on primary BC cells, or even increased MS formation among MCF-7 cells, whereas high Dkk1 concentrations decreased MS formation among both primary BC cells and MCF-7 cells.

CONCLUSIONS/SIGNIFICANCE

Our study suggests that Dkk1 treatment may be more robust than other methods for eliminating CSCs, as it challenges a general cellular homeostasis mechanism, namely, fate decision by QS. The study also suggests that low dose Dkk1 administration may be counterproductive; we showed experimentally that in some cases it can stimulate CSC proliferation, although this needs validating in vivo.

Hormones and cancer: update from Tokyo 2010

The conference took place 15 min from downtown Tokyo and talks covered a range of topics in prostate, breast and ovarian cancer. There were approximately 160 attendees from across Southeast Asia and Australia, as well as Europe and North America. Numerous high-quality posters were presented and the best abstracts were selected for short talks alongside the 20 talks from invited speakers. In this short report of the meeting, we aim to highlight some, but not all, of these talks and therefore we apologize to those that were not included in the interests of space and brevity.

Breast cancer stem cells: something out of notching?

We and others have established that the developmental Notch receptor signaling pathway is active in breast cancer cell lines, as well as in preinvasive and invasive primary samples. Recently, a role for Notch in regulating the hierarchy of stem and progenitor cells in both normal and cancer epithelium has been elucidated. Because inhibiting the Notch receptor signaling pathway is a possible future breast cancer therapy, here, we review the expression and activity of the different ligands and receptors and summarize the various ways in which the pathway's activity can be inhibited, and the likely effects of inhibition on different tumor cell subpopulations.

Comprehensive CYP2D6 genotype and adherence affect outcome in breast cancer patients treated with tamoxifen monotherapy

The association between CYP2D6 genotype and outcome in breast cancer patients treated with adjuvant tamoxifen remains controversial. We assessed the influence of comprehensive versus limited CYP2D6 genotype in the context of tamoxifen adherence and co-medication in a large cohort of 618 patients. Genotyping of 33 CYP2D6 alleles used two archival cohorts from tamoxifen-treated women with invasive breast cancer (Dundee, n = 391; Manchester, n = 227). Estimates for recurrence-free survival (RFS) were calculated based on inferred CYP2D6 phenotypes using Kaplan-Meier and Cox proportional hazard models, adjusted for nodal status and tumour size. Patients with at least one reduced function CYP2D6 allele (60%) or no functional alleles (6%) had a non-significant trend for worse RFS: hazard ratio (HR) 1.52 (CI 0.98-2.36, P = 0.06). For post-menopausal women on tamoxifen monotherapy, the HR for recurrence in patients with reduced functional alleles was 1.96 (CI 1.05-3.66, P = 0.036). However, RFS analysis limited to four common CYP2D6 allelic variants was no longer significant (P = 0.39). The effect of CYP2D6 genotype was increased by adjusting for adherence to tamoxifen therapy, but not significantly changed when adjusted for co-administration of potent inhibitors of CYP2D6. Comprehensive genotyping of CYP2D6 and adherence to tamoxifen therapy may be useful to identify breast cancer patients most likely to benefit from adjuvant tamoxifen.

Human breast cell proliferation and its relationship to steroid receptor expression

The steroidal regulation of proliferation and differentiation in the rodent mammary gland is well described, but how ovarian hormones regulate these processes in the human remains poorly understood. To investigate this, we developed the athymic nude mouse model in which intact normal human breast tissue is grafted subcutaneously and treated with estrogen and/or progesterone at human physiological serum levels. We demonstrated, first, that estrogen and not progesterone is the major epithelial cell mitogen in the adult non-pregnant, non-lactating breast, second, that estrogen induces progesterone receptor (PR) expression and, third, that PR expression is maximally induced at low estrogen concentrations while a higher amount of estrogen was required to induce proliferation. These data raised the question of whether one cell type possessed differential responses to high and low estrogen concentrations or whether PR expression and proliferation occurred in two cell populations. Using double-label immunofluorescence, we demonstrated that steroid receptor expression and cell proliferation (Ki67 antigen) occurred in separate cell populations in normal human breast epithelium, and that cells expressing the estrogen receptor-alpha (ERalpha) invariably contained the PR. We also found that this dissociation between steroid receptor expression and cell proliferation in normal epithelium was disrupted at an early stage in breast tumor formation. Recent findings presented herein support the proposal that some ERalpha/PR-positive epithelial cells are quiescent breast stem cells that act as 'steroid hormone sensors'. Such hormone sensor cells are likely to secrete positive or negative paracrine/juxtacrine factors dependent on the prevailing estrogen or progesterone concentration to influence the proliferative activity of adjacent ERalpha/PR-negative epithelial cells.

Disruption of a Quorum Sensing mechanism triggers tumorigenesis: a simple discrete model corroborated by experiments in mammary cancer stem cells

Background

The balance between self-renewal and differentiation of stem cells is expected to be tightly controlled in order to maintain tissue homeostasis throughout life, also in the face of environmental hazards. Theory, predicting that homeostasis is maintained by a negative feedback on stem cell proliferation, implies a Quorum Sensing mechanism in higher vertebrates.

Results

Application of this theory to a cellular automata model of stem cell development in disrupted environments shows a sharply dichotomous growth dynamics: maturation within 50-400 cell cycles, or immortalization. This dichotomy is mainly driven by intercellular communication, low intensity of which causes perpetual proliferation. Another driving force is the cells' kinetic parameters. Reduced tissue life span of differentiated cells results in uncontrolled proliferation. Model's analysis, showing that under the Quorum Sensing control, stem cell fraction within a steady state population is fixed, is corroborated by experiments in breast carcinoma cells. Experimental results show that the plating densities of CD44+ cells and of CD44+/24lo/ESA+ cells do not affect stem cell fraction near confluence.

Conclusions

This study suggests that stem cell immortalization may be triggered by reduced intercellular communication, rather than exclusively result from somatic evolution, and implies that stem cell proliferation can be attenuated by signal manipulation, or enhanced by cytotoxics targeted to differentiated cells. In vivo verification and identification of the Quorum Sensing mediating molecules will pave the way to a higher level control of stem cell proliferation in cancer and in tissue engineering.

Reviewers

This article was reviewed by Glenn Webb and Marek Kimmel.

Regulation of breast cancer stem cell activity by signaling through the Notch4 receptor

Notch receptor signaling pathways play an important role not only in normal breast development but also in breast cancer development and progression. We assessed the role of Notch receptors in stem cell activity in breast cancer cell lines and nine primary human tumor samples. Stem cells were enriched by selection of anoikis-resistant cells or cells expressing the membrane phenotype ESA(+)/CD44(+)/CD24(low). Using these breast cancer stem cell populations, we compared the activation status of Notch receptors with the status in luminally differentiated cells, and we evaluated the consequences of pathway inhibition in vitro and in vivo. We found that Notch4 signaling activity was 8-fold higher in stem cell-enriched cell populations compared with differentiated cells, whereas Notch1 signaling activity was 4-fold lower in the stem cell-enriched cell populations. Pharmacologic or genetic inhibition of Notch1 or Notch4 reduced stem cell activity in vitro and reduced tumor formation in vivo, but Notch4 inhibition produced a more robust effect with a complete inhibition of tumor initiation observed. Our findings suggest that Notch4-targeted therapies will be more effective than targeting Notch1 in suppressing breast cancer recurrence, as it is initiated by breast cancer stem cells.

Biomarkers of dietary energy restriction in women at increased risk of breast cancer

Dietary energy restriction (DER) reduces risk of spontaneous mammary cancer in rodents. In humans, DER in premenopausal years seems to reduce risk of postmenopausal breast cancer. Markers of DER are required to develop acceptable DER regimens for breast cancer prevention. We therefore examined markers of DER in the breast, adipose tissue, and serum. Nineteen overweight or obese women at moderately increased risk of breast cancer (lifetime risk, 1 in 6 to 1 in 3) ages between 35 and 45 were randomly allocated to DER [liquid diet, 3,656 kJ/d (864 kcal/d); n = 10] or asked to continue their normal eating patterns (n = 9) for one menstrual cycle. Biopsies of the breast and abdominal fat were taken before and after the intervention. RNA was extracted from whole tissues and breast epithelium (by laser capture microdissection) and hybridized to Affymetrix GeneChips. Longitudinal plasma and urine samples were collected before and after intervention, and metabolic profiles were generated using gas chromatography-mass spectrometry. DER was associated with significant reductions in weight [-7.0 (+/-2.3) kg] and in alterations of serum biomarkers of breast cancer risk (insulin, leptin, total and low-density lipoprotein cholesterol, and triglycerides). In both abdominal and breast tissues, as well as isolated breast epithelial cells, genes involved in glycolytic and lipid synthesis pathways (including stearoyl-CoA desaturase, fatty acid desaturase, and aldolase C) were significantly down-regulated. We conclude that reduced expressions of genes in the lipid metabolism and glycolytic pathways are detectable in breast tissue following DER, and these may represent targets for DER mimetics as effective chemoprophylactic agents.

It's all in the details: methods in breast development and cancer

The inaugural European Network for Breast Development and Cancer (ENBDC) meeting on 'Methods in Mammary Gland Development and Cancer' was held in Weggis, Switzerland last April. The goal was to discuss the details of techniques used to study mammary gland biology and tumourigenesis. Highlights of this meeting included the use of four-colour fluorescence for protein co-localisation in tissue microarrays, genome analysis at single cell resolution, technical issues in the isolation of normal and tumour stem cells, and the use of mouse models and mammary gland transplantations to elucidate gene function in mammary development and to study drug resistance in breast cancer.

Targeting inhibitor of apoptosis proteins in combination with ErbB antagonists in breast cancer

Introduction

Inhibitor of apoptosis (IAPs) proteins are a family of proteins that can block apoptosis in normal cells and have been suggested to cause resistance to apoptosis in cancer. Overexpression of oncogenic receptor tyrosine kinases is common in breast cancer; in particular 20% of all cases show elevated Her2. Despite clinical success with the use of targeted therapies, such as Trastuzumab, only up to 35% of Her2-positive patients initially respond. We reasoned that IAP-mediated apoptosis resistance might contribute to this insensitivity to receptor tyrosine kinase therapy, in particular ErbB antagonists. Here we examine the levels of IAPs in breast cancer and evaluate whether targeting IAPs can enhance apoptosis in response to growth factor receptor antagonists and TRAIL.

Methods

IAP levels were examined in a breast cancer cell line panel and in patient samples. IAPs were inhibited using siRNA or cell permeable mimetics of endogenous inhibitors. Cells were then exposed to TRAIL, Trastuzumab, Lapatinib, or Gefitinib for 48 hours. Examining nuclear morphology and staining for cleaved caspase 3 was used to score apoptosis. Proliferation was examined by Ki67 staining.

Results

Four members of the IAP family, Survivin, XIAP, cIAP1 and cIAP2, were all expressed to varying extents in breast cancer cell lines or tumours. MDAMB468, BT474 and BT20 cells all expressed XIAP to varying extents. Depleting the cells of XIAP overcame the intrinsic resistance of BT20 and MDAMB468 cells to TRAIL. Moreover, siRNA-based depletion of XIAP or use of a Smac mimetic to target multiple IAPs increased apoptosis in response to the ErbB antagonists, Trastuzumab, Lapatinib or Gefitinib in Her2-overexpressing BT474 cells, or Gefitinib in EGFR-overexpressing MDAMB468 cells.

Conclusions

The novel findings of this study are that multiple IAPs are concomitantly expressed in breast cancers, and that, in combination with clinically relevant Her2 treatments, IAP antagonists promote apoptosis and reduce the cell turnover index of breast cancers. We also show that combination therapy of IAP antagonists with some pro-apoptotic agents (for example, TRAIL) enhances apoptosis of breast cancer cells. In some cases (for example, MDAMB468 cells), the enhanced apoptosis is profound.

Normal breast tissue implanted into athymic nude mice identifies biomarkers of the effects of human pregnancy levels of estrogen

We have generated a novel model system for the study of estrogen intervention in normal breast tissue. Nulliparous human breast tissue was implanted into immunocompromised nude mice and treated with high-dose estrogen to simulate the effects of pregnancy. Treatment of mice with human mid-pregnancy levels of 17beta-estradiol for a period of 4 weeks was followed by 4 weeks of withdrawal to mimic involution. Gene expression in the xenograft tissue was then analyzed by real-time reverse transcription-PCR to identify differences between treated and control tissues. Ten genes previously identified as altered by pregnancy in rodent models were found to be differentially expressed in human breast tissue with a > or =1.8-fold up-regulation of CDC42, TGFbeta3, DCN, KRT14, LTF, and AREG and a > or =0.7-fold down-regulation of STAT1, CTGF, IGF1, and VAMP1. Immunohistochemical analysis of archival paraffin-embedded adult premenopausal human breast tissue specimens identified a significantly lower level of expression of STAT1 (P < 0.05, Mann-Whitney U test) in parous compared with age-matched nulliparous tissue (median of 24% compared with 42% epithelial cells positive). We conclude that many of the pregnancy-induced breast cancer-protective changes observed in rodent models also occur in human breast tissue following intervention using human pregnancy levels of estrogen and that STAT1 expression is a potential biomarker of parity-induced breast cancer protection in the human breast.

Resistance to endocrine therapy: are breast cancer stem cells the culprits?

From a developmental point of view, tumors can be seen as aberrant versions of their tissue of origin. For example, tumors often partially retain differentiation markers of their tissue of origin and there is evidence that they contain cancer stem cells (CSCs) that drive tumorigenesis. In this review, we summarise current evidence that breast CSCs may partly explain endocrine resistance in breast cancer. In normal breast, the stem cells are known to possess a basal phenotype and to be mainly ERalpha-. If the hierarchy in breast cancer reflects this, the breast CSC may be endocrine resistant because it expresses very little ERalpha and can only respond to treatment by virtue of paracrine influences of neighboring, differentiated ERalpha+ tumor cells. Normal breast epithelial stem cells are highly dependent on the EGFR and other growth factor receptors and it may be that the observed increased growth factor receptor expression in endocrine-resistant breast cancers reflects an increased proportion of CSCs selected by endocrine therapies. There is evidence from a number of studies that breast CSCs are ERalpha- and EGFR+/HER2+, which would support this view. CSCs also express mesenchymal genes which are suppressed by ERalpha expression, further indicating the mutual exclusion between ERalpha+ cells and the CSCs. As we learn more about CSCs, differentiation and the expression and functional activity of the ERalpha in these cells in diverse breast tumor sub-types, it is hoped that our understanding will lead to new modalities to overcome the problem of endocrine resistance in the clinic.

Lapatinib selectively inhibits HER2 over-expressing breast cancer stem-like cells

Abstract #5052

Introduction&#x2028; Standard drug treatment kills proliferating cells but recent studies show the tumourigenic cancer stem cells (CSC) are not affected, leading to recurrence. HER2 over-expression in Ductal Carcinoma In Situ (DCIS) and invasive cancer is associated with poor prognosis. We determined the effect of a dual tyrosine kinase inhibitor (Lapatinib) on CSC growth in mammosphere (non-adherent) and 3D culture in HER2 over-expressing cell lines and primary DCIS samples.&#x2028; Methods&#x2028; MCF7, BT474 (HER2+), DCIS.com and SUM225 (HER2+) cell lines were cultured as mammospheres in the presence or absence of Lapatinib (0.1-5mM). FACS analysis was used to measure proliferation (Ki67) and apoptosis (annexin / PI) rates. Western blotting was used to analyse levels phosphorylated MAPk and AKT in mammospheres. 3D matrigel culture of cell lines and primary DCIS cells was carried out with and without Lapatinib (1mM), the size and number of colonies grown were assessed. Statistical analysis was by unpaired 2-tailed T test.&#x2028; Results&#x2028; Mammosphere forming efficiency (MFE) of HER2 over-expressing but not HER2 negative cell lines was inhibited by Lapatinib (at IC50 p&lt;0.05) and the IC50 for inhibited cell lines determined (Table 1). Lapatinib treatment at the IC50 concentration in both BT474 and SUM225 reduced proliferation by 10% after 24 hours of mammosphere culture (p&lt;0.05), however there were no differences in apoptosis. Lapatinib reduced pMAPk and pAKT in HER2 over-expressing cells during mammosphere culture in a dose dependent manner. Secondary mammosphere culture demonstrated a sustained 50% reduction in mammosphere formation from primary mammospheres cultured with Lapatinib, suggesting self renewal was inhibited in mammosphere culture.&#x2028; In 3D culture of HER2 over-expressing SUM225 and primary DCIS samples Lapatinib (1µM) significantly reduced the size of the colonies but no difference was observed in the number of colonies formed (Table 1).&#x2028; &#x2028; Conclusion&#x2028; Lapatinib selectively inhibits breast cancer stem cell activity by reducing proliferation in HER2 over-expressing lines measured using non-adherent and 3D culture. These findings need to be verified by an in vivo limiting dilution tumourgenicity assay.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5052.

Tumour-promoting activity of altered WWP1 expression in breast cancer and its utility as a prognostic indicator

WWP1 is a ubiquitin ligase, associated with the post-translational regulation of several tumour-promoting and tumour suppressor proteins. Here we show that WWP1 expression is up-regulated in a subset of breast tumour cell lines and primary breast tumours. We overexpressed WWP1 in MCF10A breast epithelial cells and demonstrated increased cell growth and anchorage-independent colony formation. RNAi knockdown of WWP1 expression in T47D and MCF7 breast tumour cell lines reduced anchorage-independent colony formation. We used WWP1 protein expression levels, in combination with its sub-cellular localization, to classify breast tumours into four categories. Surprisingly, a category with low/absent WWP1 expression displayed a consistently worse prognosis compared with WWP1-expressing tumours. Importantly, the association with disease-free survival was independent of the status of other commonly used prognostic indicators. Thus, WWP1 is a prognostic marker and may be a potential therapeutic target for a subset of breast tumours.

The removal of multiplicative, systematic bias allows integration of breast cancer gene expression datasets - improving meta-analysis and prediction of prognosis

Background

The number of gene expression studies in the public domain is rapidly increasing, representing a highly valuable resource. However, dataset-specific bias precludes meta-analysis at the raw transcript level, even when the RNA is from comparable sources and has been processed on the same microarray platform using similar protocols. Here, we demonstrate, using Affymetrix data, that much of this bias can be removed, allowing multiple datasets to be legitimately combined for meaningful meta-analyses.

Results

A series of validation datasets comparing breast cancer and normal breast cell lines (MCF7 and MCF10A) were generated to examine the variability between datasets generated using different amounts of starting RNA, alternative protocols, different generations of Affymetrix GeneChip or scanning hardware. We demonstrate that systematic, multiplicative biases are introduced at the RNA, hybridization and image-capture stages of a microarray experiment. Simple batch mean-centering was found to significantly reduce the level of inter-experimental variation, allowing raw transcript levels to be compared across datasets with confidence. By accounting for dataset-specific bias, we were able to assemble the largest gene expression dataset of primary breast tumours to-date (1107), from six previously published studies. Using this meta-dataset, we demonstrate that combining greater numbers of datasets or tumours leads to a greater overlap in differentially expressed genes and more accurate prognostic predictions. However, this is highly dependent upon the composition of the datasets and patient characteristics.

Conclusion

Multiplicative, systematic biases are introduced at many stages of microarray experiments. When these are reconciled, raw data can be directly integrated from different gene expression datasets leading to new biological findings with increased statistical power.

Prolactin receptor antagonism reduces the clonogenic capacity of breast cancer cells and potentiates doxorubicin and paclitaxel cytotoxicity

Introduction

Exogenous prolactin is mitogenic and antiapoptotic in breast cancer cells, and overexpression of autocrine prolactin cDNA in breast cancer cell lines has been shown to stimulate their growth and to protect against chemotherapy-induced apoptosis. We examined the effects of the 'pure' prolactin receptor antagonist Δ1–9-G129R-hPrl (Δ1–9) on the breast cancer cell number and clonogenicity, alone and in combination with chemotherapy.

Methods

The effects of doxorubicin, paclitaxel and Δ1–9 on the growth of breast cancer cell lines (MCF-7, T47D, MDA-MB-453, MDA-MB-468 and SK-BR-3) in monolayer culture were assessed by the sulphorhodamine B assay. Effects on clonogenicity were assessed by soft agar assay for the cell lines and by the mammosphere assay for disaggregated primary ductal carcinoma in situ samples. Dual-fluorescence immunocytochemistry was used to identify subpopulations of cells expressing the prolactin receptor and autocrine prolactin.

Results

Δ1–9 as a single agent had no effect on the cell number in monolayer culture, but potentiated the cytotoxic effects of doxorubicin and paclitaxel. Doxorubicin accordingly induced expression of prolactin mRNA and protein in all five breast cancer cell lines tested. Δ1–9 alone inhibited the clonogenicity in soft agar of cell lines by ~90% and the mammosphere forming efficiency of six disaggregated primary ductal carcinoma in situ samples by a median of 56% (range 32% to 88%). Subpopulations of cells could be identified in the cell lines based on the prolactin receptor and prolactin expression.

Conclusion

Autocrine prolactin appears to act as an inducible survival factor in a clonogenic subpopulation of breast cancer cells. The rational combination of cytotoxics and Δ1–9 may therefore improve outcomes in breast cancer therapy by targeting this cell population.

Mammary stem cells and breast cancer--role of Notch signalling

Adult stem cells are found in numerous tissues of the body and play a role in tissue development, replacement and repair. Evidence shows that breast stem cells are multipotent and can self renew, which are key characteristics of stem cells, and a single cell enriched with cell surface markers has the ability to grow a fully functional mammary gland in vivo. Many groups have extrapolated the cancer stem cell hypothesis from the haematopoietic system to solid cancers, where using in vitro culture techniques and in vivo transplant models have established evidence of cancer stem cells in colon, pancreas, prostate, brain and breast cancers. In the report we describe the evidence for breast cancer stem cells; studies consistently show that stem cell like and breast cancer initiating populations can be enriched using cell surface makers CD44+/CD24- and have upregulated genes which include Notch. Notch signalling has been highlighted as a pathway involved in the development of the breast and is frequently dysregulated in invasive breast cancer. We have investigated the role of Notch in a pre-invasive breast lesion, ductal carcinoma in situ (DCIS), and have found that aberrant activation of Notch signalling is an early event in breast cancer. High expression of Notch 1 intracellular domain (NICD) in DCIS also predicted a reduced time to recurrence 5 years after surgery. Using a non-adherent sphere culture technique we have grown DCIS mammospheres from primary DCIS tissue, where self-renewal capacity, measured by the number of mammosphere initiating cells, were increased from normal breast tissue. A gamma-secretase inhibitor, DAPT, which inhibits all four Notch receptors and a Notch 4 neutralising antibody were shown to reduce DCIS mammosphere formation, indicating that Notch signalling and other stem cell self-renewal pathways may represent novel therapeutic targets to prevent recurrence of pre-invasive and invasive breast cancer.

Cyclin-dependent kinase inhibitors and basement membrane interact to regulate breast epithelial cell differentiation and acinar morphogenesis

OBJECTIVE

The cyclin-dependent kinase inhibitors (CDKIs), p21(CIP1) and p27(KIP1) regulate growth and differentiation in diverse tissue types. We aimed to determine whether p21(CIP1) or p27(KIP1) could induce a terminally differentiated phenotype in breast cells, and to examine if CDKI expression is regulated by basement membrane interactions.

MATERIALS AND METHODS

Effects of increased CDKI expression on the phenotype of MCF-10A breast epithelial cells were examined by retroviral transduction of p21(CIP1) or p27(KIP1) cDNA.

RESULTS

Overexpression of p21(CIP1) or p27(KIP1) reduced MCF-10A growth rates in monolayer cultures, altered cellular morphology and stimulated accumulation of neutral lipid droplets, suggesting partial lactational differentiation. However, markers of luminal differentiation (oestrogen and progesterone receptors, alpha-lactalbumin, beta-casein and adipophilin) were absent when examined by reverse transcriptase-polymerase chain reaction and immunohistochemistry. Cell-basement membrane contacts are known to be essential for full mammary epithelial cell differentiation and therefore parental MCF-10A cells were cultured on a basement membrane preparation (Matrigel) in which they form acini. Immunocytochemistry showed that Ki67, the cell proliferation marker, was initially expressed at high levels and as growth decreased p27(KIP1) expression steadily increased. Surprisingly, p21(CIP1) was highest at the early stages of acinus growth and was detected in proliferating cells, as demonstrated by colocalization in dual Ki67/p21(CIP1) immunofluorescence. Overexpression of p21(CIP1) or p27(KIP1) impaired formation of acini, whereas their knockdown, using siRNA, increased acinus formation.

CONCLUSION

We conclude that both p21(CIP1) and p27(KIP1) induce partial secretory differentiation of mammary cells in monolayer, but during acinus morphogenesis in 3D culture they have a highly regulated temporal expression pattern.

Origins of breast cancer subtypes and therapeutic implications

This Review summarizes and evaluates the current evidence for the cellular origins of breast cancer subtypes identified by different approaches such as histology, molecular pathology, genetic and gene-expression analysis. Emerging knowledge of the normal breast cell types has led to the hypothesis that the subtypes of breast cancer might arise from mutations or genetic rearrangements occurring in different populations of stem cells and progenitor cells. We describe the common distinguishing features of these breast cancer subtypes and explain how these features relate both to prognosis and to selection of the most appropriate therapy. Recent data indicate that breast tumors may originate from cancer stem cells. Consequently, inhibition of stem-cell self-renewal pathways should be explored because of the likelihood that residual stem cells might be resistant to current therapies.

TPD52 and NFKB1 gene expression levels correlate with G2 chromosomal radiosensitivity in lymphocytes of women with and at risk of hereditary breast cancer

PURPOSE

To evaluate a transcriptomic approach to identify healthy women at increased risk of breast cancer due to G2-radiosensitivity and look at transcripts that are differentially expressed between individuals.

MATERIALS AND METHODS

We perform the first study to assess the association of G2 radiosensitivity with basal gene expression in cultured T-lymphocytes from 11 women with breast cancer and 12 healthy female relatives using Affymetrix GeneChips.

RESULTS

Transcripts associated with radiosensitivity and breast cancer risk were predominantly involved in innate immunity and inflammation, such as interleukins and chemokines. Genes differentially expressed in radiosensitive individuals were more similarly expressed in close family members than in un-related individuals, suggesting heritability of the trait. The expression of tumour protein D52 (TPD52), a gene implicated in cell proliferation, apoptosis, and vesicle trafficking was the most strongly correlated with G2 score while nuclear factor (kappa)-B (NFKB1) was highly inversely correlated with G2 score. NFKB1 is known to be activated by irradiation and its inhibition has been previously shown to increase radiosensitivity.

CONCLUSIONS

Gene expression analysis of lymphocytes may provide a quantitative measure of radiation response potential and is a promising marker of breast cancer susceptibility.